Examples of self-propelled construction machines are, in particular, rollers, refuse compactors, road milling machines, recyclers, ground stabilizers, and stationary and mobile crushers. Such construction machines comprise an internal combustion engine as the main drive, powering the traveling drive and the drives for the working device. Working device within the scope of the present invention should be understood to refer, in particular, to a working device having a large mass and, thus, a large inertia, the speed of which can only slowly be increased from the idling speed to the operating speed. Examples thereof are the traveling drives of the self-propelled construction machines and the milling rotors of the aforementioned construction machines. During road milling operations, construction machines comprising milling rotors typically alternate between the operational mode at a slow traveling speed and the maneuvering or transportation mode at an increased traveling speed. During the operational mode, the milling rotor is lowered to a working position and is run at operating speed.
The main drive and the drives of the working device, also referred to as secondary drives, usually operate at different speeds of rotation. When working devices are connected, a defined procedure must therefore be regularly strictly adhered to, during which a predefined speed of rotation of the main drive must first be established before the connection can be made and the force flow restored. In the case of known construction machines, the gear system operates in slip mode during the coupling operation until the first driving unit and the milling rotor rotate at a synchronized speed of rotation. Depending on the difference in speed of rotation and the inertia of the drive train, the slip mode continues for a longer period of time and the wear therefore increases, resulting in a shortening of the lifespan of the components. Due to the predominant use of an internal combustion engine as the main drive unit with an output that is highly dependent on the speed of rotation, said engine does not operate within the optimal range due to the alternation between the idling and operating speeds, resulting in increased fuel consumption.
Coupling of the milling rotor is generally not possible while the driving unit is switched off or while it is running at the operating speed. It is instead necessary to reduce the speed of rotation of the driving unit, usually down to the idling speed, for the purpose of making coupling possible. Afterwards, the operating speed must be restored. For the purpose of avoiding the time-consuming coupling process when changing from maneuvering mode to operational mode, the milling rotor is often allowed to continue to run at the same speed of rotation in the maneuvering mode as is used during the operational mode. Due to the fact that the direction of rotation of the wheels or tracks of the construction machine corresponds to that of the milling rotor while reversing, there is a risk of the construction machine accelerating uncontrollably should the milling rotor unintentionally make contact with the ground. Such contact can be extremely hazardous and might also lead to damage to the milling rotor.
In the event of maintenance or installation work being carried out on the milling rotor of a milling machine, for example, when replacing milling chisels, it is necessary to move the milling rotor slowly and gradually at small angular steps or continuously, in order to allow an operator free access to the entire cylinder jacket, even when the milling rotor is fitted inside the milling machine. The first driving unit is not suitable for this purpose when said unit is the main drive. In the prior art, the use of the main drive is also not permitted for this purpose for safety reasons. It must therefore always be switched off. Such tasks are thus carried out using a second driving unit acting as a secondary drive. The known procedure involving the connection of said secondary drive whilst simultaneously switching off the main drive has been found to be relatively complex.